Wireless power receiver circuits that provide constant voltage or current to an electrical load, and methods

ABSTRACT

A wireless power receiver circuit and method for use in a wireless power transfer system are provided for providing a constant current and voltage to an electrical load, such as a chemical cell device. A wireless power receiver circuit include a first comparator circuit and a second comparator circuit configured to receive output signals output from the DC load circuit, compare the received output signal with a preselected reference voltage signal, and output first and second sub-control signals, respectively. A logical gate may generate a control signal based on a comparison of the first sub-control signal and the second sub-control signal, and feed the control signal back to a resonator circuit to control a state of an electrically-controllable switch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 16/023,925 filed Jun. 29, 2018 entitled “WIRELESS POWERRECEIVER CIRCUITS THAT PROVIDE CONSTANT VOLTAGE OR CURRENT TO ANELECTRICAL LOAD, AND METHODS,” to be or now issued as U.S. Pat. No.10,931,149, the contents of which being incorporated by reference intheir entirety herein.

The present application has subject matter that is related to subjectmatter disclosed in U.S. patent application Ser. No. 15/296,704, filedon Oct. 18, 2016, entitled “WIRELESS POWER TRANSFER TO MULTIPLE RECEIVERDEVICES ACROSS A VARIABLE-SIZED AREA,” and to subject matter disclosedin U.S. patent application Ser. No. 15/644,802, filed on Jul. 9, 2017,entitled “INTEGRATED POWER TRANSMITTER FOR WIRELESS POWER TRANSFER,”both of which are hereby incorporated by reference herein in theirentireties.

BACKGROUND

Wireless power transfer is the transmission of electrical energy from apower source to an electrical load without the use of man-madeconductors to connect the power source to the electrical load. Awireless power transfer system includes a transmitter and one or morereceiver devices. The transmitter is electrically coupled to a source ofpower and converts the power to a time-varying electromagnetic (EM)field. The one or more receiver devices receive the power via the EMfield and convert the received power back to an electric current to beutilized by an electrical load that is either part of the receiverdevice or is electrically coupled to the receiver device.

The receiver devices are configured to resonate at the characteristicfrequency at which the transmitter is operating in order to receivepower from the near EM field. The receiver devices convert the receivedpower from the near EM field into an electrical current that can then beused to power an electrical load that is part of, or that iselectrically coupled to, the receiver device.

One of the difficulties associated with current receiver devices used inwireless power transfer systems is ensuring that a constant voltage orcurrent, depending on which is needed, is provided to the electricalload. Another difficulty associated with current receiver devices usedin wireless power transfer systems is the lack of a mechanism forshutting down the resonant response of the receiver device whenelectrical power is not needed by the load. If the resonant response isnot shut down when power is not needed, the receiver device will wastepower and dissipate heat whenever the load is not using a large fractionof the available power. In known receiver devices that provide amechanism for shutting down the resonant response, the mechanismtypically includes a high-speed switching circuit implemented usingexpensive high-speed components.

A need exists for a receiver device for use in wireless power transfersystems that is capable of providing a constant current or voltage,depending on which is needed, to the load. A need exists for a receiverdevice for use in wireless power transfer systems that is capable ofshutting down the resonant response when electrical power is not neededby the load to reduce power consumption and avoid unnecessary heatdissipation.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present disclosure can be better understood withreference to the following drawings. The components in the drawings arenot necessarily to scale, with emphasis instead being placed uponclearly illustrating the principles of the disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a block diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system.

FIG. 2 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system.

FIG. 3 is a plot of the voltage across capacitor C1 shown in FIG. 2 as afunction of time demonstrating that once the RF voltage across capacitorC1 reaches a sufficiently high amplitude to begin forward-biasingrectifier diode D1 of the receiver circuit shown in FIG. 1, the receivercircuit enters a charging period.

FIG. 4 is a plot of the voltage across the load of the receiver circuitshown in FIG. 2 as a function of time.

FIG. 5 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system.

FIG. 6 is a plot of the voltage across capacitor C1 of FIG. 5 as afunction of time.

FIG. 7 is a schematic diagram of a wireless power receiver circuit inaccordance with another representative embodiment that may be used toreceive power wirelessly in a wireless power transfer system.

FIG. 8 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that provides a constant DCcurrent to the load of the wireless power receiver circuit.

FIG. 9 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that provides a constant DCcurrent to the load of the wireless power receiver circuit.

FIG. 10 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that provides a constant DCcurrent to the load of the wireless power receiver circuit.

FIG. 11 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system, and which usesn-channel MOSFET transistor as the electrically-controllable switch forshutting down the resonance response of the receiver circuit.

FIG. 12 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system, and which usesn-channel MOSFET transistor as the electrically-controllable switch forshutting down the resonance response of the receiver circuit.

FIG. 13 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system, and which usesn-channel MOSFET transistor as the electrically-controllable switch forshutting down the resonance response of the receiver circuit.

FIG. 14 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system, and which includesa chargeable battery or electrochemical cell.

FIG. 15 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system, and which includesa chargeable battery or electrochemical cell.

FIG. 16 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system, and which includea low-pass RC circuit comprising a resistor R3 and capacitor C4 forreducing ripple in the load voltage.

FIG. 17 is a schematic diagram of a wireless power PWM receiver circuitin accordance with a representative embodiment that may be used toreceive power wirelessly in a wireless power transfer system, and whichincludes a low-pass LC circuit comprising a an inductor L2 and capacitorC4 for reducing ripple in the load voltage.

FIG. 18 is a schematic diagram of a wireless power receiver circuit inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system, and which includesa linear regulator IC2 for reducing ripple in the load voltage.

FIG. 19 is a schematic diagram of a wireless power PWM receiver circuitin accordance with a representative embodiment that may be used toreceive power wirelessly in a wireless power transfer system, and whichincludes a control circuit configured to shut down the LC tank of thereceiver circuit at certain times or based on certain conditions.

FIG. 20 is a schematic diagram of a wireless power receiver circuit foruse in a wireless power transfer system in accordance with anotherrepresentative embodiment, and which includes a thermal fuse or switchthat prevents the LC tank of the receiver circuit from receiving powerif a temperature exceeds some pre-determined threshold value.

FIG. 21 is a schematic diagram of a wireless power receiver circuit foruse in a wireless power transfer system in accordance with anotherrepresentative embodiment, and which includes a user-controllable switchthat can be turned on and off by the user to allow the user to activateand deactivate the receiver circuit.

FIG. 22 is a schematic diagram of a wireless power receiver circuit foruse in a wireless power transfer system in accordance with anotherrepresentative embodiment.

FIG. 23 is a simulation of an example pulse-width modulation waveformfor a circuit topology shown in FIG. 22, where a voltage across acapacitor is plotted as a function of time.

FIG. 24 is a simulation of a voltage of a capacitor of a circuit shownin FIG. 22 plotted as a function of time.

FIG. 25 is a schematic diagram of a wireless power receiver circuit foruse in a wireless power transfer system in accordance with anotherrepresentative embodiment.

FIG. 26 is a schematic diagram of a wireless power receiver circuit withconstant-voltage and constant-current regulation in accordance withvarious embodiments.

FIG. 27 is a schematic diagram of a wireless power receiver circuit withconstant-voltage and constant-current regulation with high-side currentsensing in accordance with various embodiments.

FIG. 28 is a schematic diagram of a wireless power receiver circuit withconstant-voltage and constant-current regulation and variable voltageand current limits in accordance with various embodiments.

FIG. 29 is a schematic diagram of a wireless power receiver circuit withconstant-voltage and constant-current regulation, with high-side currentsensing, and with variable voltage and current limits in accordance withvarious embodiments.

FIG. 30 is a schematic diagram of a wireless power receiver circuit withconstant-voltage and constant-current regulation and variable voltageand current limits in accordance with various embodiments.

FIG. 31 is a schematic diagram of a wireless power receiver circuit withconstant-voltage and constant-current regulation, with high-side currentsensing, and with variable voltage and current limits in accordance withvarious embodiments.

DETAILED DESCRIPTION

In accordance with representative embodiments described herein, wirelesspower receiver circuits for use in wireless power transfer systems areprovided that provide a constant current or voltage, depending on whichis needed, to an electrical load. The wireless power receiver circuitsare configured to shut down the resonant response when electrical poweris not needed by the load to reduce power consumption and avoidunnecessary heat dissipation. Additionally, a switching device of thewireless power receiver circuit that is used for shutting down theresonant response can operate at relatively low frequencies.Consequently, the switching device can be implemented at relatively lowcost using relatively low-speed, inexpensive components.

Exemplary, or representative, embodiments will now be described withreference to the figures, in which like reference numerals representlike components, elements or features. It should be noted that features,elements or components in the figures are not intended to be drawn toscale, emphasis being placed instead on demonstrating inventiveprinciples and concepts.

In the following detailed description, for purposes of explanation andnot limitation, exemplary, or representative, embodiments disclosingspecific details are set forth in order to provide a thoroughunderstanding of inventive principles and concepts. However, it will beapparent to one of ordinary skill in the art having the benefit of thepresent disclosure that other embodiments according to the presentteachings that are not explicitly described or shown herein are withinthe scope of the appended claims. Moreover, descriptions of well-knownapparatuses and methods may be omitted so as not to obscure thedescription of the exemplary embodiments. Such methods and apparatusesare clearly within the scope of the present teachings, as will beunderstood by those of skill in the art. It should also be understoodthat the word “example,” as used herein, is intended to benon-exclusionary and non-limiting in nature.

The terminology used herein is for purposes of describing particularembodiments only, and is not intended to be limiting. The defined termsare in addition to the technical, scientific, or ordinary meanings ofthe defined terms as commonly understood and accepted in the relevantcontext.

The terms “a,” “an” and “the” include both singular and pluralreferents, unless the context clearly dictates otherwise. Thus, forexample, “a device” includes one device and plural devices. Relativeterms may be used to describe the various elements' relationships to oneanother, as illustrated in the accompanying drawings. These relativeterms are intended to encompass different orientations of the deviceand/or elements in addition to the orientation depicted in the drawings.

The term “constant voltage,” as that term is used herein, means that thevoltage is substantially constant in that it does not vary over time bymore than about 10% from its average value. Similarly, the term“constant current,” as that term is used herein, means that the currentis substantially constant in that it does not vary over time by morethan about 10% from its average value. Representative embodimentsdescribed herein are directed to a receiver device that delivers aconstant voltage or a constant current, depending on which is needed ordesired, to an electrical load that is electrically coupled to thereceiver device.

Where a first device is said to be connected or coupled to a seconddevice, this encompasses examples where one or more intermediate devicesmay be employed to connect the two devices to each other. In contrast,where a first device is said to be directly connected or directlycoupled to a second device, this encompasses examples where the twodevices are connected together without any intervening devices otherthan electrical connectors (e.g., wires, bonding materials, etc.). Thephrase “electrically coupled to,” as that phrase is used herein, candenote a wireless electromagnetic coupling between two devices or awired connection between two devices, with or without intermediatedevices employed to interconnect the two devices.

FIG. 1 is a block diagram of a wireless power receiver circuit 1 thatmay be used to receive power wirelessly in a wireless power transfersystem, such as, for example, the wireless power transfer system 100disclosed in the aforementioned U.S. application Ser. No. 15/296,704(hereinafter “the '704 application) to wirelessly receive power from awireless power transmitter. In FIG. 1, the wireless power receivercircuit 1 is a pulse-width-modulated (PWM) wireless power receivercircuit represented by a combination of four functional blocks 3-6, eachof which performs one or more functions and has at least one input andone output.

Block 3 is an LC resonator circuit comprising at least a capacitor 7 andan inductor 8 connected in parallel, and an electrically-controllableswitch 9. The electrically-controllable switch 9 is activated by anoutput signal of block 6, as will be described below in more detail. Inaccordance with this representative embodiment, when theelectrically-controllable switch 9 is in an opened state, the LCresonator circuit receives RF power from an ambient magnetic field andoutputs RF power. The RF power output from block 3 is fed into an inputterminal of block 4, which is an RF rectifier circuit that rectifies theRF power, converting it into DC power. In FIG. 1, the RF rectifiercircuit is represented by a diode 11. The DC power output from block 4is fed into an input terminal of block 5, which comprises an energystorage device 12 connected in parallel with the DC load 2. Forillustrative purposes, the energy storage device 12 is represented inFIG. 1 by a single capacitor.

Block 5 outputs an output signal at an output terminal thereof to block6. The output signal is proportional to a parameter (either voltage orcurrent) that is intended to be regulated so as to remain substantiallyconstant. As will be described below in more detail, in some cases thewireless power receiver circuit 1 is configured to deliver asubstantially constant DC current to the DC load 2, and in other casesthe wireless power receiver circuit 1 is configured to deliver asubstantially constant DC voltage to the DC load 2.

The output signal from block 5 is fed as an input signal into an inputterminal of block 6, which comprises a comparator 14 having apreselected amount of hysteresis and a reference terminal 15. Thecomparator 14 compares the input signal to a reference signal receivedat the reference terminal 15 and outputs a control signal having a valuethat is based on whether the input signal is greater than the referencesignal or is less than or equal to the reference signal. The controlsignal output from block 6 is fed into a control signal input terminal16 of block 3. As indicated above, the state of theelectrically-controllable switch 9 is controlled (i.e., activated ordeactivated) based on the value of the control signal output from block6 and input to block 3. The electrically-controllable switch 9 may beany voltage-controlled switch, such as, for example, a metal oxidesemiconductor field effect transistor (MOSFET), a relay, a complementarymetal oxide semiconductor (CMOS), an RF switch, etc.

The closed feedback loop comprising the four blocks 3-6 acts to regulatethe load parameter measured by the signal that is output from block 5 soas to ensure that either a substantially constant current or voltage isdelivered the DC load 2, even if the load impedance or the ambientmagnetic field strength are varied.

The circuit 1 has two states depending on the state of theelectrically-controllable switch 9: idle state (switch 9 is in a firststate, e.g., switch 9 is closed); active state (switch 9 is in a secondstate, e.g., switch 9 is open). In accordance with this representativeembodiment, the first state of switch 9 is the closed state and thesecond state of switch 9 is the opened state and the idle and activestates of the receiver circuit 1 correspond to the closed and openedstates, respectively, of the switch 9. It should be noted that theopposite could be true in other embodiments. For purposes ofdemonstrating the inventive principles and concepts, the followingdiscussion assumes that the circuit 1 is in the active state when switch9 is in the opened state and is in the idle state when switch 9 is inthe closed state.

When the circuit 1 is in the active state, switch 9 is turned off (theopened state), and the LC resonator circuit 3 receives power due to theinduced voltage, Vind, causing an RF voltage to appear across capacitor7. There will be an initial ring-up period as the LC resonator circuit 3builds up energy. Once the RF voltage across capacitor 7 reaches asufficiently high amplitude to begin forward-biasing a rectifier circuit4, the circuit 1 enters a charging period. The rectifier circuit 4rectifies the RF voltage across capacitor 7 and charges the capacitor 12of block 5, causing its DC voltage to slowly rise. When the voltage onthe non-inverting input terminal of comparator 14 exceeds the voltagereference, VREF, the output of the comparator 14 switches to its maximumoutput value (e.g., a logic level High), which causes theelectrically-controllable switch 9 to turn on (the closed state). Atthis point, the circuit 1 enters the idle state.

When the switch 9 is turned on, it shorts the capacitor 7 to ground andthe RF voltage across capacitor 7 drops to a very low level. Therectifier diode 11 is then reverse-biased, and prevents the capacitor 12from discharging through the inductor 8. The capacitor 12 slowlydischarges through the load 21, causing the voltage across the capacitor12 to drop. The comparator 14 is designed to have a preselected amountof hysteresis so that it does not immediately change state when thevoltage on the non-inverting input terminal of comparator 14 begins todrop. Once the voltage on the non-inverting input terminal of capacitorC3 29 drops by a certain amount, ΔV, set by the hysteresis, thecomparator 14 changes state and outputs its minimum output value (e.g.,a logic level low). At this point, the electrically-controllable switch9 is turned off. The circuit 1 re-enters the active state, and the cyclerepeats.

It can be seen from the above description that the LC resonator circuitis shut down during the idle period, and does not receive power ordissipate heat. This allows the receiver circuit 1 to operate in a widedynamic range of ambient magnetic field strengths. In a weak magneticfield, the receiver circuit 1 will be in the active state for a largepercentage of the PWM cycle. In a strong magnetic field, the receivercircuit 1 will be in the idle state for a large percentage of the PWMcycle. In the idle state, the LC resonator circuit is detuned, ornon-resonant. It therefore has a very weak and/or negligible resonantresponse to the ambient magnetic field, and will not receive power ordissipate heat. In other words, the resonant response of the receivercircuit 1 is shut down during the idle period to prevent the receivercircuit 1 from receiving power and from dissipating heat. The lack ofheat dissipation during the idle period improves both the safety and theefficiency of the receiver circuit 1.

Each of the blocks 3-6 of the wireless power receiver circuit 1 can havea variety of configurations. Several examples of wireless power receivercircuits that perform the functions and that have the features describedabove with reference to FIG. 1 will now be described with reference toFIGS. 2-21. It should be noted that the inventive principles andconcepts are not limited to the examples shown in FIGS. 2-21 and that avariety of wireless power receiver circuits not specifically shown anddescribed herein can be created that have the features and perform thefunctions described above with reference to FIG. 1. It should also benoted that the wireless power receiver devices disclosed herein are notlimited to being used with any particular wireless power transmitter,but may be used with any suitable wireless power transmitter and in anysuitable wireless power transfer system.

FIG. 2 is a schematic diagram of a wireless power receiver circuit 20 inaccordance with a representative embodiment that may be used to receivepower wirelessly in a wireless power transfer system, such as, forexample, the wireless power transfer system 100 disclosed in the '704application. In accordance with this representative embodiment, thewireless power receiver circuit 20 is a PWM wireless power receivercircuit that provides a constant voltage to an electrical load 21. Anambient magnetic field drives the inductor, L1 22, and induces avoltage, Vind, represented in FIG. 2 by an RF voltage source 23 inseries with inductor L1 22. Assuming B denotes the component of theambient magnetic field that is parallel to the dipole moment of theinductor, L1 22, the capacitor, C1 24, in combination with inductor, L122, forms a resonant LC tank circuit that is tuned to resonate at thefrequency of oscillation of the ambient magnetic field, B.

The circuit 20 has two states depending on the state of anelectrically-controllable switch, S1 26: active (switch S1 26 is off)and idle (switch S1 26 is on). The switch S1 26 may be anyvoltage-controlled switch, such as, for example, one or more MOSFETtransistors, a relay, one or more CMOS transistors, an RF switch, etc.When the circuit 20 is in the active state, switch S1 26 is turned off,and the tank circuit comprising L1 22 and C1 24 receives power due tothe induced voltage, Vind, causing an RF voltage to appear acrosscapacitor, C1 24. There will be an initial ring-up period as the LC tankcircuit builds up energy.

FIG. 3 is a plot of the voltage across capacitor C1 24 as a function oftime. Once the RF voltage across capacitor C1 24 reaches a sufficientlyhigh amplitude to begin forward-biasing a rectifier diode, D1 27, thecircuit 20 enters a charging period. The rectifier diode D1 27 rectifiesthe RF voltage across C1 24 and charges a capacitor C3 29, causing itsDC voltage to slowly rise. Two resistors, R1 31 and R2 32, form aresistive voltage divider that provides a fraction of the voltage acrosscapacitor C3 29 to the non-inverting input terminal of a comparator, IC133. When the voltage on the non-inverting input terminal of comparatorIC1 33 exceeds the voltage reference, VREF, the output of the comparatorIC1 33 will switch to logic level high. The electrically-controllableswitch S1 26 will then be turned on. At this point, the circuit 20enters the idle state.

When the switch S1 26 is turned on, it shorts the capacitor C1 24 toground. The switch S1 26 preferably has low resistance to ensure thatthe Q of the LC tank circuit is very low, and the RF voltage acrosscapacitor C1 24 drops to a very low level. The rectifier diode D1 27 isthen reverse-biased, and prevents the capacitor C3 29 from dischargingthrough the inductor L1 22. The capacitor C3 29 slowly dischargesthrough the load 21, causing its voltage to drop.

In accordance with this representative embodiment, the comparator IC1 33is designed to have a preselected amount of hysteresis so that it doesnot immediately change state when the voltage on the non-inverting inputterminal of comparator IC1 33 begins to drop. Once the voltage on thenon-inverting input terminal of capacitor C3 29 drops by a certainamount, ΔV, set by the hysteresis, the comparator IC1 33 changes stateand outputs a logic level low. At this point, theelectrically-controllable switch S1 26 is turned off. The circuit 20re-enters the active state, and the cycle repeats.

It can be seen from the above description that the LC tank circuit isshut down during the idle period, and does not receive power ordissipate heat. This allows the receiver circuit 20 to operate in a widedynamic range of ambient magnetic field strengths. In a weak magneticfield, the receiver circuit 20 will be in the active state for a largepercentage of the PWM cycle. In a strong magnetic field, the receivercircuit 20 will be in the idle state for a large percentage of the PWMcycle. In the idle state, the resonator, i.e., the LC tank, is detuned,or non-resonant. It therefore has a very weak and/or negligible resonantresponse to the ambient magnetic field, and will not receive power ordissipate heat. In other words, the resonant response of the receivercircuit 20 is shut down during the idle period to prevent the receivercircuit 20 from receiving power and from dissipating heat. The lack ofheat dissipation during the idle period improves both the safety and theefficiency of the receiver circuit 20.

FIG. 4 is a plot of the voltage across the load 21 as a function oftime. The voltage across the load 21 is regulated, and is substantiallyconstant over time, with a small triangle-wave ripple. The voltagevaries only slightly over time between about 19.9 v and 21.7 v. In thisexample, the average voltage is 20.8 v. The voltage across the load 21varies by less than 10%, and typically less than 5%, from the averageload voltage. The amplitude of the ripple is set by the hysteresis ofthe comparator IC1 33 and may be made smaller by choosing a comparatorwith less hysteresis.

As indicated above, in accordance with this representative embodiment,the comparator IC1 33 is designed to have a preselected amount ofhysteresis so that it does not immediately change state when the voltageon the non-inverting input terminal of comparator IC1 33 begins to drop.Rather, the comparator IC1 33 does not change state and output a logiclevel low until after the voltage on the non-inverting input terminal ofcapacitor C3 29 has dropped by a certain amount, ΔV, set by thehysteresis. This feature allows the frequency of the switching of thereceiver circuit 20 between the active and idle states to besignificantly lower than the frequency of oscillation of the ambientmagnetic field. For example, the switching frequency of the receivercircuits in accordance with the inventive principles and concepts can be10,000 or even 100,000 times lower than that of the ambient magneticfield. Consequently, the components of the receiver circuit 20 that areinvolved in the switching process can be relatively slow speed,relatively inexpensive, components.

In contrast, known PWM receiver circuits do not incorporate apreselected amount of hysteresis in the comparator sufficient to allowthem to operate at a substantially lower frequency than the frequency ofoscillation of the ambient magnetic field. Consequently, they performswitching at a frequency that is comparable to the frequency ofoscillation of the ambient magnetic field. Because this frequency can berelatively high, the circuit elements that are involved in the switchingprocess are also relatively high-speed components, which are typicallymore complex and expensive than low-speed components.

FIG. 5 is a schematic diagram of a wireless power PWM receiver circuit40 in accordance with a representative embodiment that may be used toreceive power wirelessly in a wireless power transfer system. Thewireless power PWM receiver circuit 40 shown in FIG. 4 is identical tothe wireless power PWM receiver circuit 20 shown in FIG. 2, except thatthe receiver circuit 40 shown in FIG. 5 includes an additionalcapacitor, C0 41, in series with the inductor, L1 22, and an additionalrectifier diode D2 42 in parallel with capacitor C1 24. The operation ofthe receiver circuit 40 is largely the same as the operation of thereceiver circuit 20 shown in FIG. 2, except that the ratio of C1 24 toC0 40 is preselected to more effectively match the impedance of the LCtank circuit comprising C1 24, L1 22 and C0 41 to that of the load 24.

FIG. 6 is a plot of the voltage across capacitor C1 24 as a function oftime. The purpose of the rectifier diode D2 42 is to provide a DC pathfor the load current, since the capacitor C0 41 blocks this DC currentfrom flowing through the inductor, L1 22. The rectifier diode D2 42causes a DC charge to build up on the capacitor C1 24 during the activestate, which can be seen in the waveform plotted in FIG. 6.

FIG. 7 is a schematic diagram of a wireless power PWM receiver circuit50 in accordance with another representative embodiment that may be usedto receive power wirelessly in a wireless power transfer system. The PWMreceiver circuit 50 is identical to the PWM receiver circuit 40 shown inFIG. 5, except that the location of the switch S1 26 has been changed.The operation of the receiver circuit 50 is largely the same as theoperation of the receiver circuit 20 shown in FIG. 2.

If the capacitance ratio, C1 24/C0 41, is too large, then the receivercircuit 40 shown in FIG. 5 will not be able to sufficiently detune theresonant LC tank circuit when the PWM circuit 40 is in the idle state.If the LC tank circuit is not sufficiently detuned, the induced voltage,Vind, will generate a large circulating RF current that will dissipateheat. This will not only cause a waste of power, but may also causethermal issues in the receiver circuit 40. The receiver circuit 50 shownin FIG. 7 solves this problem by directly shorting the inductor L1 22 toground, rather than just shorting the capacitor C1 24 to ground. Thisguarantees that the LC tank circuit is detuned from resonance in theidle state, regardless of the ratio of C1 to C0.

The three PWM receiver circuits 20, 40 and 50 described above withreference to FIGS. 2, 5 and 7, respectively, are all designed to providea constant DC voltage to the load 21. However, there are some situationsin which a load requires a constant DC current instead of a constant DCvoltage. FIGS. 8, 9 and 10 show wireless power PWM receiver circuits 60,70 and 80, respectively, in accordance with representative embodimentsthat provide a constant DC current to the load 21.

In the PWM receiver circuits 60, 70 and 80 shown in FIGS. 8, 9 and 10,respectively, the load 21 is placed in series with the resistor R1 31.The comparator IC1 33 provides feedback that keeps the voltage acrossresistor R1 31 very close to the reference voltage, Vref. Because thecurrent through resistor R1 31 is equal to the current through the load21, these circuits will force the current through the load 21 to remainconstant.

The electrically-controllable switch S1 26 shown in FIGS. 2, 5 and 7-10may be implemented with, for example, an n-channel MOSFET transistor, Q192, as shown in the FIGS. 11, 12 and 13. FIGS. 11, 12 and 13 areschematic diagrams of wireless power PWM receiver circuits 90, 100 and110, respectively, in accordance with a representative embodiment thatmay be used to receive power wirelessly in a wireless power transfersystem. The PWM receiver circuits 90, 100 and 110 are very similar tothe PWM receiver circuits shown in FIGS. 2, 5 and 7-10, except that theswitch S1 26 has been replaced with the n-channel MOSFET transistor Q192. The operations of the receiver circuits 90, 100 and 110 are largelythe same as the operation of the receiver circuit 20 shown in FIG. 2 todeliver a constant DC voltage to the load 21.

In the receiver circuits 90, 100 and 110, the n-channel MOSFETtransistor Q1 92 plays the role of the electrically-controllable switch9 described above with reference to FIG. 1. However, unlike an idealswitch, the MOSFET transistor Q1 92 can still conduct current when it isin its off-state due to the MOSFET's internal body diode, which pointsfrom the source to the drain. Therefore, in order for the MOSFET Q1 92to operate as a switch, the voltage of the drain cannot be allowed tobecome more negative than one diode-drop below the voltage of thesource. To achieve this condition, an additional capacitor, C2 91, hasbeen placed in series with the MOSFET Q1 92 in the receiver circuits 90and 110 shown in FIGS. 11 and 13, respectively. When the circuits are inthe active state, the capacitor C2 91 builds up a DC charge that keepsthe drain of MOSFET Q1 92 positive relative to the source. The value ofthe capacitor C2 91 should be chosen to be sufficiently large that itbehaves as an RF short at the resonant frequency of the LC tank circuit.

The capacitor C2 91 is not used in the receiver circuit 100 shown inFIG. 12. In the receiver circuit 100, the capacitor C1 24 builds up a DCcharge and plays the role that the capacitor C2 91 plays in the othertwo receiver circuits 90 and 110.

FIG. 14 is a schematic diagram of a wireless power PWM receiver circuit120 in accordance with a representative embodiment that may be used toreceive power wirelessly in a wireless power transfer system. The PWMreceiver circuit 120 is identical to the PWM receiver circuit 20 shownin FIG. 2, except that the receiver circuit 120 includes a battery orelectrochemical cell, represented by B1 121 in FIG. 14. The operation ofthe receiver circuit 120 is largely the same as the operation of thereceiver circuit 20 shown in FIG. 2, except that the receiver circuit120 charges the battery or electrochemical cell B1 121.

The battery or electrochemical cell B1 121 is in parallel with thecapacitor C3 29 and effectively behaves like a very large capacitance.Consequently, the receiver circuit 120 can be made to have a very lowPWM frequency. When the voltage of the battery or electrochemical cellB1 121 is low, the receiver circuit 120 is in the active state, and thebattery or electrochemical cell B1 121 is continually charged with aconstant DC current. The magnitude of the constant DC current depends onthe amplitude of Vind, which is proportional to the strength of theambient magnetic field. In stronger fields, the charging current will behigher, and the battery or electrochemical cell B1 121 will charge morequickly.

Once the voltage of the battery or electrochemical cell B1 121 becomeshigh enough to trigger the comparator, IC1 33, theelectrically-controllable switch S1 26 is turned on, and the circuit 120enters the idle state. When the receiver circuit 120 is in the idlestate, the battery or electrochemical cell B1 121 continually dischargesthrough the load 21, causing its voltage to drop. Once the voltage onthe battery or electrochemical cell B1 121 drops by a certain amount,determined by the hysteresis of the comparator IC1 33, theelectrically-controllable switch S1 26 is deactivated, and the circuit120 re-enters the active state.

It should be noted that the constant-voltage PWM circuit topologiesshown in FIGS. 2, 5 and 7 will all work equally well as battery chargingcircuits if the battery or electrochemical cell B1 121 is placed inparallel with the capacitor C3 29 in those circuits, and if the powerlevel is chosen such that the DC charging current does not exceed themaximum allowable charging current for the battery or electrochemicalcell B1 121.

FIG. 15 is a schematic diagram of a wireless power PWM receiver circuit130 in accordance with another representative embodiment that may beused to receive power wirelessly in a wireless power transfer system. Inaccordance with this representative embodiment, the receiver circuit 130charges the battery or electrochemical cell B1 121 with acurrent-limiting resistor R3 131. If the DC charging current of thereceiver circuit 120 shown in FIG. 14 exceeds the maximum allowedcharging current for the battery or electrochemical cell B1 121, oneoption is to modify the circuit 120 by adding the current-limitingresistor R3 131 in series with the battery or electrochemical cell B1121, as shown in FIG. 15.

The receiver circuit 130 shown in FIG. 15 has the additional benefitthat the combination of the battery or electrochemical cell B1 121 andcurrent-limiting resistor R3 131 behaves as a low-pass filter thatsubstantially smooths the voltage of the load 21 and eliminates thetriangle-wave ripple on the load voltage shown in FIG. 4.

It should be noted that the PWM frequency of the receiver circuit 130shown in FIG. 15 is determined by the value of the capacitor C3 29,unlike the receiver circuit 120 shown in FIG. 14, in which the PWMfrequency is determined by the capacity of the battery orelectrochemical cell B1 121. This means that the PWM frequency of thereceiver circuit 130 shown in FIG. 15 can be much higher than the PWMfrequency of the receiver circuit 120 shown in FIG. 14.

With reference again to FIG. 4, as indicated above, the periodicswitching of the receiver circuit 20 causes a small triangle-wave rippleon the voltage across capacitor C3 29. The amplitude of this ripple isentirely set by the hysteresis of the comparator 33, and may be madesmaller or larger by choosing a comparator with smaller or largerhysteresis, respectively. However, the hysteresis cannot be eliminated,as this hysteresis, in part, determines the frequency of the PWM. If theload 21 is connected in parallel with capacitor C3 29, then the load 21will also have the same triangle-wave ripple on its voltage.

In some applications, this ripple may be undesirable. The receivercircuit 130 shown in FIG. 15 has the property that the combination ofresistor R3 131 and the battery or electrochemical cell B1 121 act as alow-pass filter that smooths the voltage across the load 21. The sameeffect may be achieved using a low-pass RC or LC filter without abattery, as will now be described with reference to FIGS. 16 and 17.

FIG. 16 is a schematic diagram of a wireless power PWM receiver circuit140 in accordance with a representative embodiment that may be used toreceive power wirelessly in a wireless power transfer system. Inaccordance with this representative embodiment, the receiver circuit 140includes a low-pass RC circuit comprising resistor R3 131 and capacitorC4 141. The low-pass RC circuit comprising resistor R3 131 and capacitorC4 141 reduces ripple in the load voltage.

FIG. 17 is a schematic diagram of a wireless power PWM receiver circuit150 in accordance with a representative embodiment that may be used toreceive power wirelessly in a wireless power transfer system. Inaccordance with this representative embodiment, the receiver circuit 150includes a low-pass LC circuit comprising inductor L2 151 and capacitorC4 141. The low-pass LC circuit comprising resistor inductor L2 151 andcapacitor C4 141 reduces ripple in the load voltage.

It is also possible to use a linear regulator to provide a constantvoltage to the load, as will now be described with reference to FIG. 18.FIG. 18 is a schematic diagram of a wireless power PWM receiver circuit160 in accordance with a representative embodiment that may be used toreceive power wirelessly in a wireless power transfer system. Inaccordance with this representative embodiment, the receiver circuit 160includes a linear regulator IC2 161 that reduces ripple in the loadvoltage.

It may be desirable in some applications to shut down the LC tank of thereceiver circuit resonance at certain times or based on certainconditions. An example of the manner in which shutting down the LC tankof the receiver circuit can be performed will now be described withreference to FIG. 19.

FIG. 19 is a schematic diagram of a wireless power PWM receiver circuit170 in accordance with a representative embodiment that may be used toreceive power wirelessly in a wireless power transfer system. Inaccordance with this representative embodiment, the receiver circuit 170is configured to shut down the LC tank comprising inductor L1 22 andcapacitor C1 24 to prevent receiver circuit resonance at certain timesor based on certain conditions. The receiver circuit 170 has an OR-gate,IC2 171, which activates switch S1 26 whenever a logic-level high signalis received from the comparator, IC1 33, or from a control circuit IC3173, which may be, for example, a microcontroller integrated circuit(IC) chip, an RF communications chip, an optical communications chip, athermometer chip with a binary output based on a temperature threshold,etc.

In some situations, it is necessary or desirable to protect the PWMreceiver circuit or load against excessive heat. For example, it ispossible for the LC tank of the PWM receiver circuit to absorb power andgenerate excessive heat in the case where the circuit is exposed to anunusually high magnetic field strength, or in the case of failure of thecomparator IC1 33. FIG. 20 is a is a schematic diagram of a wirelesspower PWM receiver circuit 180 for use in a wireless power transfersystem in accordance with another representative embodiment thatprovides such thermal protection. A thermal fuse, F1 181, is placed inseries with the resonant LC tank circuit comprising capacitor C1 24 andinductor L1 22. When the fuse F1 181 is exposed to a temperatureexceeding some pre-determined threshold (TH) value, it becomes anopen-circuit (placed in the off state), thereby preventing the LC tankcircuit from receiving any power. When the fuse F1 181 is in the offstate, the resonant response of the resonant circuit is shut down suchthat the receiver circuit 180 receives only a minimum amount, if any, ofelectrical power. Otherwise, the fuse F1 181 is in a PWM state.

In some cases, the thermal fuse F1 181 may be replaced by a thermalswitch that returns to normal operation once the temperature has fallenbelow the pre-determined TH value. Thus, element 181 represents athermal fuse or a thermal switch. It should be noted that the thermalfuse or switch F1 181 may be placed in series with the LC tank circuitin any of the PWM topologies described above with reference to FIGS. 2,5 and 7-19.

FIG. 21 is a schematic diagram of a wireless power receiver circuit 190for use in a wireless power transfer system in accordance with anotherrepresentative embodiment, and which includes a user-controllable switchS2 191 that can be turned on and off by the user to allow the user toactivate and deactivate the receiver circuit 190. In all other respects,the receiver circuit 190 is identical to the receiver circuit 20 shownin FIG. 2. When the switch S2 191 is open (placed in the off state), theresonant response of the receiver circuit 190 is shut down such that thereceiver circuit 190 is incapable of receiving any power, regardless ofthe strength of the ambient magnetic field. The receiver circuit 190 istherefore rendered safe, or protected, from any sort of over-voltage orover-temperature fault when the switch S2 191 is in the off positionshown in FIG. 21.

Another schematic for a wireless power receiver circuit 200 is shown inFIG. 22. Specifically, FIG. 22 includes a circuit schematic for apulse-width modulated receiver with constant voltage regulationaccording to various embodiments. An ambient magnetic field drives aninductor L1, and induces a voltage, V_(ind), represented by a sinusoidalRF voltage source in series with inductor L1. The resistor Rp representsthe parasitic resistance of the inductor L1. The capacitors C1, C2, andC3, in combination with inductor L1, form a resonant LC tank circuit 205which is tuned to resonate at the frequency of oscillation of theambient magnetic field.

Voltage Regulation. The wireless power receiver circuit 200 has twostates depending on the state of an electronically-controllableswitching element, such as MOSFET Q1. The first state is an active statein which MOSFET Q1 is off, and the second state is an idle state inwhich the MOSFET Q1 is on. When the wireless power receiver circuit 200is in the active state, MOSFET Q1 is turned off and the LC tank circuit205, comprising L1, C1, C2, and C3, receives power due to the inducedvoltage, Vinci, causing an RF voltage to appear across capacitor C1. Asmay be appreciated, there will be an initial ring-up period as the LCtank circuit builds up energy. During this time, the diode D1, willcause capacitor C1 to accumulate charge.

Once the peak voltage across capacitor C1 reaches a predeterminedamplitude (e.g., a sufficiently high amplitude to begin forward-biasingthe diode D2), the wireless power receiver circuit 200 enters a chargingperiod. The rectifier diodes D1 and D2 rectify and double the RF voltageacross capacitor C1 and charge the capacitor C5, causing the DC voltageof capacitor C5 to slowly rise. During this time, the peak voltageacross capacitor C1 is limited by diode D2 to be no more than one diodedrop above the voltage of capacitor C5, and the minimum voltage acrosscapacitor C1 is limited by diode D1 to be no lower than one diode dropbelow zero, which is shown by the voltage waveform of the chargingperiod of FIG. 23. Specifically, FIG. 23 shows a simulation of anexample pule-width modulation waveform for the topology of the wirelesspower receiver circuit 200 shown in FIG. 22. The voltage acrosscapacitor C1 is plotted as a function of time.

Referring back to FIG. 22, the two resistors R1 and R2 form a resistivevoltage divider which provides a fraction of the voltage across C5 tothe non-inverting input of the comparator IC1. During this period, an RFvoltage will be present on the drain of Q1. The body diode of Q1 willrectify this RF voltage and charge capacitor C4 until no more RF currentis conducted. The ratio of C2 to C1 may be chosen to ensure that thepeak voltage present on the drain of Q1 does not exceed its maximumrating. Note that in some cases, this condition may be satisfied withoutthe need for capacitor C3, in which case capacitor C3 may be omitted andreplaced by a short. Otherwise, in those cases where capacitor C3 isneeded, capacitor C4 may be omitted and replaced by a short.

When the voltage of the non-inverting input of IC1 exceeds the voltagereference VREF the comparator's output will switch to logic level high.The MOSFET Q1 will then be turned on. At this point the wireless powerreceiver circuit 200 enters the idle state. When the MOSFET Q1 is turnedon, it places capacitor C4 in parallel with capacitors C1 and C2. If theMOSFET Q1 has low resistance, and capacitor C4 has low reactance, thiswill significantly detune the LC tank circuit 205, and the RF currentcirculating in the resonator will drop to a very low level. During theidle period, the rectifier diodes D1 and D2 are reverse-biased and donot conduct.

The capacitor C5 slowly discharges through the voltage regulator VR1into the load, causing its voltage to drop. The comparator IC1 isdesigned to have a certain amount of hysteresis so that it does notimmediately change state when the voltage of the non-inverting inputbegins to drop. Once the voltage of the capacitor C5 drops by a certainamount, ΔV, set by the hysteresis, the comparator IC1 changes state andoutputs a logic level low. At this point, the MOSFET Q1 is turned off.The circuit re-enters the active state, and the cycle repeats.

FIG. 24 shows the voltage of capacitor C5 as a function of time.Specifically, FIG. 24 shows a simulation of the voltage of capacitor C5in the wireless power receiver 200 circuit 200 shown in FIG. 22, plottedas a function of time. The wireless power receiver circuit 200 keeps thevoltage of capacitor C5 between 20V and 22V. This voltage range may befreely chosen by changing the values of resistors R1 and R2, the valueof the reference voltage, VREF, and the size of the hysteresis of thecomparator IC1. Note the timescale is longer than that of FIG. 23 inorder to show multiple PWM cycles.

Further, FIG. 24 contains a small triangle-wave ripple. The voltage ofC5 rises during the active period, and falls during the idle period. Theamplitude of the triangle-wave ripple is determined by the hysteresis ofthe comparator IC1 and the ratio of resistor R2 to resistor R1. In manycases, the amplitude of the voltage ripple of capacitor C5 is largerthan the maximum permissible ripple of the load voltage. Therefore, thelinear regulator VR1 is used to provide a smoother DC output voltage tothe load. The regulator may be chosen such that the difference betweenthe minimum voltage of capacitor C5 and the output voltage of theregulator is higher than the regulator's drop-out voltage.

However, the power dissipated by the voltage regulator VR1 will beproportional to the voltage difference between its input and output.Therefore, the values of resistors R1 and R2 may be chosen such that theminimum voltage difference between the input and the output of theregulator VR1 is just high enough to exceed its drop-out voltage, but nohigher.

Current Regulation. FIG. 25 shows a wireless power receiver circuit 210similar to that shown in FIG. 22, except a load 220 is placed in serieswith resistor R1. This causes the voltage on the non-inverting input ofIC1 to be proportional to the current through the load, rather than thevoltage across capacitor C5. Therefore, the wireless power receivercircuit 210 shown in FIG. 25 will generate a pulse-width modulationwaveform that keeps the load current between an upper and a lower bound,set by the hysteresis of comparator IC1. Other than this, the principalsof operation are the same as described above.

Voltage and Current Regulation. FIG. 26 shows a wireless power receivercircuit 215 that combine the voltage and current regulation functions ofthe circuits in FIG. 22 and FIG. 25. In other words, FIG. 26 shows acircuit schematic for a pulse-width modulated receiver withconstant-voltage and constant-current regulation. Similar to FIGS. 22and 25, the wireless power receiver circuit 215 includes an LC tankcircuit 205 comprising capacitors C1, C2, and C3, as well as inductorL1.

A logic gate, such as an OR-gate IC3, turns on the MOSFET Q1 when thevoltage across capacitor C5 exceeds a certain threshold, defined byresistors R2 and R1, or when the current through the load exceeds acertain limit, set by resistor R3. In effect, resistor R3 acts as acurrent sensing resistor.

Accordingly, as shown in FIG. 26, a wireless power receiver circuit 215is described as comprising a resonator circuit configured to resonate ata frequency of an ambient magnetic field generated by a wireless powertransmitter of a wireless power transfer system when the wireless powerreceiver circuit 215 is in an active state and to output an outputsignal. The resonator circuit includes an electrically-controllableswitch, such as MOSFET Q1 or similar device, that is controllable by acontrol signal to cause the resonator circuit to switch from a firststate to a second state, and vice versa.

The wireless power receiver circuit 215 may include an AC-to-DCrectifier circuit, where the RF output signal output from the resonatorcircuit is input to the AC-to-DC rectifier circuit. The AC-to-DCrectifier circuit may convert the RF output signal into a DC powersignal and output the DC power signal.

The wireless power receiver circuit 215 may include a DC load circuithaving a DC load 220 and an energy storage device (e.g., a battery orchemical cell), where, during the active state, the DC power signaloutput by the AC-to-DC rectifier circuit charges the energy storagedevice, causing a substantially constant DC current or voltage to bedelivered to the DC load.

Further, the wireless power receiver circuit 215 includes a firstcomparator circuit (e.g., comparator IC2) that that receives an outputsignal output from the DC load circuit, compares the received outputsignal with a preselected reference voltage signal, and outputs a firstsub-control signal. The first comparator circuit may have a first inputcoupled to a line, for instance, between the negative terminal of the DCload 220 and the current sensing resistor R3. The first comparatorcircuit may have a second input coupled to a reference voltage.

The wireless power receiver circuit 215 further includes a secondcomparator circuit (e.g., comparator IC1) that receives an output signaloutput from the DC load circuit, compares the received output signalwith the preselected reference voltage signal, and outputs a secondsub-control signal. The second comparator circuit may have a first inputcoupled to a line, for instance, between the resistors R1 and R2. Thesecond comparator circuit may have a second input coupled to a referencevoltage, which may be the same as or similar to the reference voltageinput to the first comparator circuit.

The wireless power receiver circuit 215 may also include a logicalelement, such as an OR gate IC3, configured to generate the controlsignal based on a comparison of the first sub-control signal and thesecond sub-control signal, the control signal being fed back to theresonator circuit to control the state of the electrically-controllableswitch (e.g., MOSFET Q1). While the embodiments described herein relateto an OR gate IC3, it is understood that other logical gates orequivalent circuits may be employed.

The functionality of the circuit of FIG. 26 allows the circuit to drivea constant-current load while also protecting the load and the circuitfrom over-voltage if the load is disconnected. This may be beneficialwhen the load includes a rechargeable chemical cell, such as a battery.Usually when charging batteries or other chemical cells, a recharge isrequired to be performed with a constant current until a cell reaches aconstant voltage. Accordingly, the wireless power receiver circuit 215of FIG. 26 is beneficial when recharging chemical cells, such asbatteries, as it will automatically turn off when current is exceededand/or voltage are exceeded.

FIG. 27 shows an embodiment of a wireless power receiver circuit 225that combines the foregoing voltage and current regulation functions,like that shown in FIG. 26, except the load current is detected using aseries resistor R3 on a high-side. Current monitor IC4 acts as ahigh-side current monitor, which measures the voltage difference acrossresistor R3 and outputs a voltage proportional to the voltagedifference. The comparator IC2 compares a voltage output by the currentmonitor IC4 to a predetermined reference voltage, as chosen. The valueof resistor R3, the gain of current monitor IC4, and the referencevoltage of comparator IC2 set the current limit, as may be appreciated.

Variable Voltage and/or Current Limits. Because the voltage and currentlimits of the wireless power receiver circuits 215, 225 shown in FIGS.26 and 27, respectively, are proportional to the voltage reference(VREF) inputs to comparator IC1 and comparator IC2, one or both of thesereference voltages can be made variable in order to provide electroniccontrol of the voltage and/or current limits, as shown in the wirelesspower receiver circuits 230, 235 shown in FIGS. 28 and 29, respectively.

Specifically, the reference voltages are produced by the output ofanalog control circuit(s) 250 a, 250 b (collectively “analog controlcircuits 250”). The analog control circuits 250 may produce an analogvoltage output which is proportional to the desired voltage or currentlimit. The control inputs to the analog control circuits 250 may beeither analog or digital. The control input may be produced by a varietyof sources, such as a microcontroller, a potentiometer, an analogsensing and feedback circuit, etc.

Load Control with Variable Voltage and/or Current Limits. As shown inwireless power receiver circuits 240, 245 of FIGS. 31 and 32,respectively, an electrically controllable switch S2 may be inserted inseries with the load 220 in order to turn the load on or off. The switchmay be implemented using a P-channel MOSFET, a PNP transistor, or anyother electrically-controllable switching device as may be appreciated.

It should be emphasized that the above-described embodiments of thepresent disclosure are merely possible examples of implementations setforth for a clear understanding of the principles of the disclosure.Many variations and modifications may be made to the above-describedembodiments without departing substantially from the spirit andprinciples of the disclosure. All such modifications and variations areintended to be included herein within the scope of this disclosure andprotected by the following claims.

Therefore, the following is claimed:
 1. A wireless power receivercircuit for use in a wireless power transfer system, the wireless powerreceiver circuit comprising: a resonator circuit configured to resonateat a frequency of an ambient magnetic field generated by a wirelesspower transmitter of the wireless power transfer system when thewireless power receiver circuit is in an active state and to output aradio frequency (RF) output signal, the resonator circuit including anelectrically-controllable switch that is controllable by a controlsignal to cause the resonator circuit to switch from a first state to asecond state, and vice versa; an alternating current (AC)-to-directcurrent (DC) rectifier circuit, the RF output signal output from theresonator circuit being input to the AC-to-DC rectifier circuit, theAC-to-DC rectifier circuit converting the RF output signal into a DCpower signal and outputting the DC power signal; a DC load circuithaving a DC load and an energy storage device, wherein during the activestate the DC power signal output by the AC-to-DC rectifier circuitcharges the energy storage device, causing a substantially constant DCcurrent or voltage to be delivered to the DC load; a first comparatorcircuit that receives an output signal output from the DC load circuit,compares the received output signal with a preselected reference voltagesignal, and outputs a first sub-control signal; a second comparatorcircuit that receives an output signal output from the DC load circuit,compares the received output signal with the preselected referencevoltage signal, and outputs a second sub-control signal; and a logicalgate configured to generate the control signal based on a comparison ofthe first sub-control signal and the second sub-control signal, thecontrol signal being fed back to the resonator circuit to control thestate of the electrically-controllable switch.
 2. The wireless powerreceiver circuit of claim 1, wherein the DC load circuit comprises arechargeable chemical cell battery.
 3. The wireless power receivercircuit of claim 1, wherein the logical gate is an OR gate.
 4. Thewireless power receiver circuit of claim 1, further comprising anelectrically-controllable switch in series with the DC load circuitconfigured to turn the DC load circuit on or off.
 5. The wireless powerreceiver circuit of claim 1, wherein: the preselected reference voltagesignal provided to the first comparator circuit is a first variablereference voltage signal output by a first analog control circuit, thefirst analog control circuit being provided with a current limit controlinput; and the preselected reference voltage signal provided to thesecond comparator circuit is a second variable reference voltage signaloutput by a second analog control circuit, the second analog controlcircuit being provided with a voltage limit control input.
 6. Thewireless power receiver circuit of claim 1, wherein the wireless powerreceiver circuit further comprises a current monitor circuit acting as ahigh-side current monitor, and configured to measure a voltagedifference across a resistor in series with the DC load circuit, andoutput a voltage proportional to the voltage difference to the firstcomparator.
 7. The wireless power receiver circuit of claim 1, wherein:the first and second states of the electrically-controllable switch areclosed and opened states, respectively, of the electrically-controllableswitch; switching the resonator circuit from the closed state to theopened state causes the wireless power receiver circuit to switch froman idle state to the active state, and vice versa; a resonant responseof the resonator circuit is shut down when the wireless power receivercircuit is in the idle state; and when the wireless power receivercircuit is in the idle state, a minimum amount of electrical power isreceived by the wireless power receiver circuit.
 8. The wireless powerreceiver circuit of claim 4, wherein: when the output signal output fromthe DC load circuit has dropped to a value that is a predeterminedamount below the preselected reference voltage signal, the controlsignal generated by the comparator circuit has a low value that causesthe electrically-controllable switch to switch from the closed state tothe opened state, thereby placing the wireless power receiver circuit inthe active state.
 9. The wireless power receiver circuit of claim 5,wherein: the comparator circuit exhibits a preselected amount ofhysteresis that determines the predetermined amount below thepreselected reference voltage signal that the output signal output fromthe DC load circuit drops before the comparator circuit outputs thecontrol signal; the preselected amount of hysteresis is preselected todelay switching of the wireless power receiver circuit from the idlestate to the active state; and the delay ensures that the switching ofthe wireless power receiver circuit is at a frequency that is lower thanthe frequency of the ambient magnetic field.
 10. The wireless powerreceiver circuit of claim 1, wherein: when the energy storage device isfully charged, the wireless power receiver circuit switches from theactive state to an idle state, and wherein when the wireless powerreceiver circuit is in the idle state, the resonator circuit is shutdown and has no resonant response to the ambient magnetic field; when anelectrical charge on the energy storage device drops below apredetermined level, the wireless power receiver circuit switches fromthe idle state to the active state; and when the wireless power receivercircuit is in the active state, the resonator circuit has a resonantresponse to the ambient magnetic field.
 11. A method for receivingwireless power in a wireless power transfer system, the wireless powerreceiver circuit comprising: with a resonator circuit of a wirelesspower receiver circuit, operating in an active state or in an idlestate, wherein in the active state, the resonator circuit resonates at afrequency of an ambient magnetic field generated by a wireless powertransmitter of the wireless power transfer system and outputs a radiofrequency (RF) output signal, the resonator circuit including anelectrically-controllable switch that is controllable by a controlsignal to cause the resonator circuit to switch from a first stateduring which the resonator circuit is in the idle state to a secondstate during which the resonator circuit is operating in the activestate, and vice versa; with an alternating current (AC)-to-directcurrent (DC) rectifier circuit, converting the RF output signal into aDC power signal and outputting the DC power signal; with a DC loadcircuit having a DC load and an energy storage device, charging theenergy storage device when the resonator circuit is in the active stateto cause a substantially constant DC current or voltage, depending onwhich is used by the DC load, to be delivered to the DC load; and with afirst comparator circuit, receiving an output signal output from the DCload circuit, comparing the received output signal with a preselectedreference voltage signal, and outputting a first sub-control signal;with a second comparator circuit, receiving an output signal output fromthe DC load circuit, comparing the received output signal with thepreselected reference voltage signal, and outputting a secondsub-control signal; and with a logical gate, generating the controlsignal based on a comparison of the first sub-control signal and thesecond sub-control signal, and feeding the control signal back to theresonator circuit to control the state of the electrically-controllableswitch.
 12. The method of claim 11, wherein the DC load circuitcomprises a rechargeable chemical cell battery.
 13. The method of claim11, wherein the logical gate is an OR gate.
 14. The method of claim 11,further comprising, with an electrically-controllable switch in serieswith the DC load circuit, turning the DC load circuit on or off.
 15. Thewireless power receiver circuit of claim 1, further comprising:providing, with a first analog control circuit, the preselectedreference voltage signal to the first comparator circuit as a firstvariable reference voltage signal, the first analog control circuitbeing provided with a current limit control input; and providing, with asecond analog control circuit, the preselected reference voltage signalto the second comparator circuit as a second variable reference voltagesignal, the second analog control circuit being provided with a voltagelimit control input.
 16. The wireless power receiver circuit of claim 1,further comprising measuring, with a current monitor circuit acting as ahigh-side current monitor, a voltage difference across a resistor inseries with the load circuit, and outputting, with the current monitorcircuit, a voltage proportional to the voltage difference to the firstcomparator.
 17. The wireless power receiver circuit of claim 1, wherein:the first and second states of the electrically-controllable switch areclosed and opened states, respectively, of the electrically-controllableswitch; switching the resonator circuit from the closed state to theopened state causes the wireless power receiver circuit to switch froman idle state to the active state, and vice versa; a resonant responseof the resonator circuit is shut down when the wireless power receivercircuit is in the idle state; and when the wireless power receivercircuit is in the idle state, a minimum amount of electrical power isreceived by the wireless power receiver circuit.
 18. The method of claim14, wherein: when the output signal output from the DC load circuit hasdropped to a value that is a predetermined amount below the preselectedreference voltage signal, the control signal generated by the comparatorcircuit has a low value that causes the electrically-controllable switchto switch from the closed state to the opened state, thereby placing thewireless power receiver circuit in the active state.
 19. The method ofclaim 15, wherein: the comparator circuit exhibits a preselected amountof hysteresis that determines the predetermined amount below thepreselected reference voltage signal that the output signal output fromthe DC load circuit drops before the comparator circuit outputs thecontrol signal; the preselected amount of hysteresis is preselected todelay switching of the wireless power receiver circuit from the idlestate to the active state; and the delay ensures that the switching ofthe wireless power receiver circuit is at a frequency that is lower thanthe frequency of the ambient magnetic field.
 20. The method of claim 11,wherein: when the energy storage device is fully charged, the wirelesspower receiver circuit switches from the active state to an idle state,and wherein when the wireless power receiver circuit is in the idlestate, the resonator circuit is shut down and has no resonant responseto the ambient magnetic field; when an electrical charge on the energystorage device drops below a predetermined level, the wireless powerreceiver circuit switches from the idle state to the active state; andwhen the wireless power receiver circuit is in the active state, theresonator circuit has a resonant response to the ambient magnetic field.